Being the crucial pieces of equipment they are, brakes are always on the forefronts of fleet managers’ minds when it comes to upgrades and maintenance. With the continued steady growth of disc brake adoption on both tractors and trailers, you may have some questions, like whether to use disc brakes or drum brakes on your trailers; whether using discs on the tractor and drums on the trailer, or vice versa, is a good idea; or why air disc brake penetration is lower for trailers than it is for tractors. Fleet Equipment contacted brake manufacturers for the answers.

Disc vs. drum

Hendrickson

Any fleet debating which brakes to spec must decide between disc brakes and drum brakes. Mechanically, there are many differences between the two, but for fleet managers, the debate comes down to this: Drum brakes are significantly less expensive, but disc brakes provide better performance. So the choice becomes: Do you go with the performance or the cost? We put this question to various brake experts, and the answers were fairly conclusive.

“There are cases where drum brakes make sense for fleets, just like air disc brakes make sense for others,” begins Keith McComsey, director of marketing and customer solutions for Bendix Spicer Foundation Brake LLC. “Conditions such as fleet operation, application and how long they keep trailers must be considered. Each fleet will review their choices for a return on their investment in their equipment based upon their trailer specifications.

“However,” he continues, “for approved applications, if we are talking about safety and long-term total cost of ownership [TCO], air disc brakes make the most sense. This is due to their contribution to stopping power and opportunities to improve the bottom line—including increasing payload, extending service intervals, lowering maintenance costs, reducing CSA violation potential and eliminating rust jacking risks.”

Marty Watterson, Meritor’s field service manager for the trailer division, offers similar advice. “Depending on how a fleet uses its tractors and trailers, it is commonly accepted that disc brakes have overall better operating performance than drum brakes in optimal conditions,” he says. “Disc brakes dissipate heat better than a drum, keeping brake pads from overheating and causing breakdown. Serviceability is easier on disc brakes, as there are fewer components and less labor time is needed. When combining the benefits of better friction wear, decreased service intervals, less maintenance time and increased uptime, the total cost of ownership for disc brakes versus drum brakes is much lower. This applies to both tractors and trailers.”

“The biggest advantages of air disc brakes [ADBs] include reduced stopping distances and reduced overall total maintenance costs over the life of the trailer due to longer service/maintenance intervals and reduced change-out time,” explains Bill Hicks, director of product planning—Americas with SAF-Holland Inc. “In addition, the unique design of air disc incorporating the automatic adjustment mechanism integral to the caliper creates a more robust assembly that is less likely to create an out-of-service or adjustment condition.”

Hicks notes that many fleets have begun spec’ing wider drum brakes on their trailers to close the performance gap between drum and disc brakes. However, the added weight and cost of the wider brakes negates some of the overall benefits of choosing this approach.

“The wider drums still suffer from drum brake fade,” Hicks says. “The fundamental design differences between disc and drum are why we see fleets moving to the overall performance improvement of an ADB system.”

“S-Cam drum brakes have a long history and are well understood by service technicians and fleet maintenance management,” notes Jeff Geist, director of engineering with Stemco Brake Products. “Air disc brakes offer improved stopping performance and ease of friction replacement, but maintenance and service procedures are new and different, and the experience base is less populated.”

Geist points out that tractors are more likely to be serviced by OEM dealerships trained in the new technology, whereas trailers are often serviced by independent shops more familiar with S-Cam drum brakes.

“So the selection of brake system type is really based on the fleet’s requirements and experience,” he says. “Many fleets are running both brake types to accumulate their own data to determine true lifecycle and cost per mile comparisons.”

Mixing and matching

SAF-Holland

Many fleets mix and match their brakes—using disc brakes on the tractor and drum brakes on the trailer or vice versa. This has become a popular concept since the passing of reduced stopping distance (RSD) laws, as it provides a balance of stopping distance and lower cost. But is this a good idea? And what effect does it have on things like stopping distance and overall brake performance? Our panel of brake experts weighs in.

“Tractor/trailer brake balance is definitely a concern when different braking technologies are paired,” Stemco’s Geist explains. “S-Cam drum brakes inherently have a self-energizing behavior where the lead shoe pulls into the drum when making contact. This creates additional braking force that increases effectiveness early in the braking event. Air disc brakes tend to operate in a more linear way with pressure. TMC PR628C includes a segment on matching tractor and trailer brake torque output at low application pressures where the majority of braking events occur.”

SAF-Holland’s Hicks also stresses that proper brake balance is critical to minimizing stopping distances. “A properly balanced system allows each axle on the tractor-trailer combination to exert the maximum designed brake force,” he notes. “If the brake system on the tractor or trailer is over- or under-spec’d, brake balance will be affected. In this situation, one or more of the axles will not be able to exert its maximum brake force, and the overall stopping distance could increase.”

Jeff Wittlinger, business unit director of wheel-end and brake systems for Hendrickson, points out the importance of understanding the differences between braking characteristics for panic stops versus everyday stops. Wittlinger says that it’s easy to focus on panic stops because of their implications for safety, but everyday braking represents the vast majority of braking situations and ultimately is what wears brakes out over time.

“Careful consideration of maintenance intervals and costs is required for fleets to understand how certain brake combinations affect total cost of ownership,” he says. “Hendrickson works closely with fleets to understand inputs that affect brake performance like duty cycle, driver inputs, operating environment, tractor-trailer balance and maintenance practices to provide a customized braking recommendation from tractor through trailer.”

“Meritor encourages the use of split systems, particularly for the cost conscious fleet,” Watterson says. “Split system specifications do not reduce overall performance for the vehicle. However, it is important to ensure that all tractor and trailer brakes are serviced to the original OEM specifications.”

Bendix’s McComsey notes that while any combination of disc and drum brakes between the tractor and trailer will provide adequate performance and meet FMVSS stopping requirements, ADBs on tractor and trailer are the optimal combination.

“This will provide the greatest level of brake performance by virtually eliminating brake fade, providing the shortest stopping distances and extending brake friction life compared to drum brakes,” he says.

Adoption rates

Bendix

Despite the general consensus that air disc brakes are better for performance, adoption rates for ADBs, though increasing, remain low for tractors—and growth is even slower for trailer brakes. So why is that?

Meritor’s Watterson offers up a simple explanation. “Traditionally, an average fleet purchases and operates more trailers than tractors each year,” he notes. “Therefore, the initial investment cost for disc brakes on tractors has been lower than on trailers. However, as the price of disc brakes continues to become cost-competitive and the awareness of the technology and benefits of disc brakes is more widely shared, we expect more fleet owners to spec disc brakes on both tractors and trailers.”

Like Watterson, Hendrickson’s Wittlinger sees the longer trade cycles for trailers versus tractors (seven years or more for trailers as opposed to five years or less for tractors) as the main culprit behind the slower adoption of disc brakes on trailers.

“Air disc brake and wheel-end maintenance costs will be different for first owners, and fleets will need to weigh these factors carefully to determine whether ADB is a good fit for their fleet,” he says. “In the end, the disc brake has proven to be a more effective product for this wheel position, so this has been driving penetration on tractors.”

“The industry has already arrived at a point where air disc brakes are standard on certain truck models,” SAF-Holland’s Hicks says. “This trend will likely continue to expand as more OEMs respond to market demand. The trailer OEMs have been more hesitant; however, we foresee new van and specialty models being brought to market with ADBs as standard and drums as a deduct option.”

Hicks says that the transition for ADBs to become standard for all truck and trailers is dependent upon the fleet as the truck and trailer OEMs will meet their demands driven by each fleet’s cost/benefit analysis related to ADB. “As market penetration continues to grow and economies of scale come further into play for suppliers,” he says, “the additional cost for ADBs will continue to be reduced, accelerating continued penetration of ADB into the market.”

Stemco’s Geist notes that fleets generally find it easier to tolerate the added cost and mass of ADB systems on tractors rather than trailers. “Trailers, except for specialty units, tend to be more cost- and weight-sensitive,” he points out. “Every pound that is eliminated from the vehicle can be converted into revenue-producing cargo. Lifecycle costs of ADBs are also still being evaluated and can vary greatly depending on vocation and other variables. As experience with ADB applications and performance is gained and compared with S-Cam brakes, the best value proposition will prevail.”
Meritor

Bendix’s McComsey says that tractor units tend to get fully utilized, but some fleets have more trailers than tractors, meaning that several trailers are sitting each day, lengthening the time between brake replacements, which could account, in part, for the slower adoption.

That said, McComsey notes that Bendix sees increased growth of ADB adoption on trailers as fleets increasingly value the benefits in brake performance and reduced TCO.

For this growing market, Bendix recently announced the ADB22X-LT, an air disc brake designed specifically with trailers in mind.

“We optimized the caliper and carrier design for weight, taking out roughly 40 lbs. on a tandem-axle trailer while maintaining durability requirements for the trailer market,” McComsey explains. “As part of the new trailer brake, Bendix also designed a new ADB pad that extends the life of the pad beyond current ADB standards. This new pad can also be used on the tractor when doing pad replacements, which allows fleets to stock a single set of pads for both tractor and trailer.”

Application considerations

As with all equipment, knowing your application and your fleet’s specific needs is crucial. Aspects like the duty-cycle, the climate and operating conditions, the axle capacity and the gross vehicle weight all have an impact on what kind of brakes a tractor/trailer will require.

“When choosing the correct brake for a vehicle combination, it is important to size the brake and actuation to match the load carrying capabilities of the axle,” says Hendrickson’s Wittlinger. “Brake ratings are determined by the selection of several components, including brake model and lining, brake chamber size and stroke and slack adjuster length. For optimum performance, brakes should be selected based upon the maximum required load and static loaded radius that is needed for the application. Hub, suspension, axle and tire ratings may all differ on a given trailer, and it is the minimum rated component of this group that determines the gross axle weight ratings (GAWR).

“Fleets running LTL will experience different performance and brake life than fleets that run to their full GCW,” says Bendix’s McComsey. “Also, operations that run 24/7—such as some tank applications—will achieve unique results as well, so brake duty-cycles are unique from fleet to fleet. I would suggest that fleets contact Bendix to discuss their application needs to help determine the best brake setup.”

“Each fleet will need to determine its demand and usage for equipment over the predicted life cycle,” says Roger Jansen, product manager of suspension/axle/brake systems—Americas with SAF-Holland. “Fleets with applications that have a higher duty-cycle or demand on their braking systems will likely benefit the most by upgrading their trailer brakes, whether to wider drum brakes or to air disc brakes.”

Jansen recommends asking your ADB supplier for a TCO ADB evaluation spreadsheet to help predict the return on investment related to the ADB purchase.